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Introduction

Figure 1 – Location of Pancho Villa State Park.

Pancho Villa State Park in Columbus, New Mexico, (Fig. 1) was
established in 1959 “in interest of preservation of the memory of
the unique, historical occasion of the last hostile action by foreign
troops within the continental United States” (Kottlowski, 1980). It
became the only park in the United States to be named after a foreign
invader. The creation of the 60-acre park was a gesture of
good will between the United States and Mexico. The town of
Columbus has been designated a National Historic Site. Across the
street from the park is the privately owned Pancho Villa Museum.

Geology

Pancho Villa State Park lies on the nearly flat alluvial plain
extending southward from the Tres Hermanas Mountains (Spanish
for three sisters). The alluvial plain consists of gravel, sand, and
mud derived from the adjacent mountains and transported by
streams and sheetwash. The Mimbres River, which starts in the
Pinos Altos Mountains to the north, flowed episodically throughout
the Pleistocene and Holocene past Deming, around the Florida
Mountains, east of Columbus, and into Mexico to fill the playa
lakes south of Las Palomas in the Bolson de los Muertos
(Kottlowski, 1980; Love and Seager, 1996). Today, the Mimbres
River flows into the Mimbres Basin east of Deming. Ground water
in the buried alluvial fan deposits, in part a remnant of the ancient
Mimbres River, is pumped to irrigate fields in the Deming and
Columbus areas.

Figure 2 –Diagram of formation of the Rio Grande rift

The most prominent land feature within the park is Villa Hill
(also known as Coote’s Hill), in the northwest corner. This
hill rises approximately 25 ft above the flat, featureless desert floor
and is composed of reddish-brown to black, vesicular basalt. Gas
bubbles formed the vesicles or irregular cavities during cooling of
the basaltic lava as it flowed from its vent, now covered by the
lava. Small xenoliths or fragments of limestone and other country
rock are found within the basalt. A sample of the basalt was dated
by the 40Ar/39Ar dating technique and was determined to be 3.8 million years (m.y.) old (unpublished age determination, New
Mexico Geochronology Research Laboratory, New Mexico
Institute of Mining and Technology). A sample of a similar lava
flow southwest of Columbus has been dated as 3 m.y. old (Seager
et al., 1984). Geochemically, the Pancho Villa basalt is alkaline and
similar in chemical composition to the basalts found in the Potrillo
volcanic field south of Las Cruces and east of Columbus. The
basalt was formed by partial melting of material derived from the
upper mantle (Anthony et al., 1992) and was extruded during the
latest stages of the formation of the Rio Grande rift (Seager et al.,
1984; McLemore, 1999). A rift is a long, narrow feature where the
Earth’s crust is being pulled apart by plate tectonic forces (Fig. 2). As the
crust is pulled apart, large blocks in the center of the rift drop into
the resulting elongate depression or trough, known as a graben.
The Rio Grande rift is actually a series of linked troughs extending from near Alamosa, Colorado, southward to El Paso, Texas. The
Mimbres Basin east of Deming is one of these down-dropped
troughs. The remainder of the state park consists of relatively flat,
sandy desert soil; small sand dunes are common throughout the
park.

The Tres Hermanas Mountains are three prominent peaks 7 miles
northwest of the park. The three peaks consist predominantly of a
quartz-monzonite stock that is about 35 m.y. old (unpublished age
determination, New Mexico Geochronology Research Laboratory,
New Mexico Institute of Mining and Technology). The quartz
monzonite is surrounded by a thick sequence of predominantly
Paleozoic and Cretaceous sedimentary rocks and Tertiary volcanic
rocks (Balk, 1962; Griswold, 1961; Leonard, 1982). Fragments of
monzonite, rhyolite, latite, basalt, andesite, chert, and limestone,
some as large as 1–2 ft in length, were transported by occasional
stream flows from the Tres Hermanas Mountains to the park where
they may be seen scattered among the cacti. Gold, silver, copper,
lead, and zinc were discovered in the Tres Hermanas Mountains in
about 1881. Total production from the mineral deposits in the district
is unknown but is estimated from 1885 to 1957 as $600,000
worth of copper, gold, silver, lead, and zinc, including 200,000 lbs
of lead and 1 million lbs of zinc (McLemore et al., 1996).

The Florida Mountains form the jagged peaks north to northeast of Columbus. The range consists of Paleozoic through lower
Tertiary sedimentary and volcanic rocks overlying Proterozoic and
Cambrian granite and syenite plutons (Clemons and Brown, 1983;
Clemons, 1984). Tertiary rhyolite, diorite, and andesite intrude the
older rocks. From 1880 to 1956, 5,000 lbs copper, <10 oz gold, 8,000
oz silver, and >30,000 lbs lead worth approximately $102,000 were
produced from carbonate-hosted lead–zinc replacement and polymetallic
vein deposits in the Florida Mountains mining district. In
addition, 200 short tons of fluorite and 1,421 long tons of 22–30%
manganese have been produced from epithermal veins.

The sharp, jagged peaks of the Big Hatchet Mountains lie on the
western skyline. The Big Hatchet Mountains consist of faulted and
tilted Paleozoic limestones and Cretaceous shales and sandstones
that show few signs of mineralization or alteration (Zeller, 1975;
Drewes et al., 1988; Drewes, 1991a, b). The rocks in the area consist
predominantly of Horquilla Limestone and Earp Formation, with
a thin, thrusted band of Oligocene andesite or basaltic andesite.
Small carbonate-hosted lead–zinc replacement deposits have been
identified along the faults; production from these deposits has
been small, amounting to less than $2,000 (McLemore et al., 1996).